It has previously been proposed in U.S. Pat. No. 4,131,834 to Blaszkowski, issued Dec. 26, 1978, to provide moisture detectors based upon measuring changes in electrical conductivity between spaced electrodes which sense rain when the gap between such electrodes is bridged by the rainwater. However, the amount of conductivity varies with atmospheric contaminants in the water as well as corrosion and wear of the electrical contacts forming the electrodes. Therefore such moisture detectors do not provide accurate measurement of the amount of moisture present.
In addition, moisture detectors have been proposed for detecting moisture based on measuring the changing capacitance in the gap between spaced electrodes due to changes in the dielectric material of such gap, such as when water is present. However, such a moisture detector suffers from poor sensitivity due to the proximity effects of moving wiper blades on such capacitance and from interfering electrical fields from power lines and other sources.
It has also been proposed to detect moisture by sensing the sound created by infringing droplets but this is inaccurate and is unable to detect light mists or fog accumulations. Similarly, moisture detectors based upon measurement of the mass changes due to the presence of water droplets are insensitive to light mist or fog.
Some optical detectors have sensed moisture based upon the interruption of light beam by the water droplets. However, these detectors also are insensitive to gradual accumulations of moisture as mist or fog. Also, windshield wipers interrupt the light beam and require gating mechanisms to disable the light detector during wiper sweeps so they are somewhat impractical.
It is believed that moisture detectors which sense water droplets by light refraction within the droplets are a substantial improvement over these moisture detectors. However, previously optical detectors which detect raindrops based upon light refraction have suffered from several disadvantages, including small detecting area, low sensitivity, error signals due to ambient light, and dependence upon long-term stability of light sources and photo-detectors whose characteristics change significantly with temperature, aging, operating point, and supply voltage variations.
The optical detection method and apparatus of the present invention overcomes these problems using a first light beam transmitted through a large area of the window and by employing a mask which prevents the first light beam from directly reaching the photo-detector unless such light beam is refracted by water droplets on the outer surface of the windshield. As a result, the output signal of the photo-detector indicating the presence of water droplets is zero when no droplets are present and increases in amplitude with the size and amount of water droplets present on the windshield of the vehicle or other window for a more accurate and more sensitive measurement of the accumulation of rain on such window.
A second light source may be provided for measuring fog by reflecting a second light beam off the inner surface of the window to the photo-detector in order to detect fog on such inner surface. As a result of diffusion of the second light beam by the fog less light is reflected off of the window to the photo-detector so that the fog measurement signal decreases in amplitude with increasing amounts of fog. The output signal of the photo-detector for measuring the accumulation of fog is distinguished from that for measuring the accumulation of rain by operating the two light sources at different times such as by electronically switching the inputs of two current amplifiers driving such light sources in an alternating manner to the output of a single oscillator. A third light source directly radiates light upon the photo-detector to bias it to the proper operating point. A narrowband amplifier tuned to the oscillator frequency is connected to the output of the photo-detector transistor to amplify the rain and fog measurement signals. The output of such amplifier is connected through a negative feedback circuit to the third light source to cancel gain changes produced by changes in ambient light, temperature changes and aging of the light source and photo-transistor, and power supply variations.
It has been previously proposed in U.S. Pat. No. 5,059,877 to Teder, issued Oct. 22, 1991, to operate a windshield wiper on an automobile automatically by the optical detection of water droplets on the windshield using light reflection from the outer surface of the windshield. An accumulation of raindrops on such outer surface scatters or diffuses the light beam and reduces the output signal of the photo-detector with increases in raindrop accumulation. The photo-detector is a photo-transistor which is coupled to the windshield by a light pipe of small diameter which greatly reduces the measured area of the windshield to less than approximately 1 sq. cm. This reduces the sensitivity of measurement, especially to a small accumulation of raindrops. The optical detector system of the present invention solves these problems by using light refraction with a masking device in front of the photo-detector and a wider light beam which covers a much larger area of the windshield, over 31 sq. cm. This larger measurement area greatly improves the accuracy of measurement of the amount of accumulated rainfall. Also, the present invention operates in a more efficient manner by refracting the light beam with the water droplets to redirect it toward the photo-detector which is shielded from direct radiation of such light beam by the masking device. As a result the output signal of the photo-detector increases with an increase in the amount of raindrops thereby improving its sensitivity. In addition, the Teder rain measurement system is more sensitive to changes in ambient light levels and therefore requires that a compensation circuit sample and store the ambient light level signals for subtraction from the measurement signal. Also, high ambient light levels including bright sunlight or at night when the headlights of an approaching car strike the windshield at a light intensity greater than predetermined limits cause the raindrop detection and wiper operation process to be suspended temporarily. This ambient light problem is avoided in the optical detector of the present invention by employing oscillator pulsed light sources, a narrowband amplifier at the output of the photo-detector tuned to the oscillator frequency and negative feedback from the output of such amplifier through a bias light source directed at the photo-detector.
U.S. Pat. No. 4,867,561 to Fujii et al., issued Sep. 19, 1989, also shows a similar optical detector for detecting rain by light reflection from the windshield in a detection area of extremely small size of less than 2 sq. cm. The photo-detector is two-dimensional array of photo-electric transducer elements mounted within an optical system housing supported beneath the dashboard closely adjacent the windshield. This optical detector employs light reflection for sensing raindrops on the outer surface of the windshield so that the presence of the raindrops reduces the amount of light which is reflected to the photo-detector and thereby reduces the output signal of such photo-detector. As a result the Fujii detector system has limited sensitivity and reduced accuracy compared to that of the present invention. Ambient light level changes are also a problem with this detector. Thus the ambient light level is measured and used to reduce the threshold levels of the comparators in the detection circuit for measuring rain and fog in an attempt to reduce inaccuracies due to change in the ambient light level. Also no measurements may be made if excessive ambient light is present such as bright sunlight.
A similar teaching is also shown in U.S. Pat. No. 4,595,866 to Fukatsu et al., issued Jun. 17, 1986, which relates to an optical detector for detecting rain on the windshield by the transmission of light from an external light source outside the windshield to a photo-detector within the automobile. The light beam is transmitted directly to the photo-detector, so that the output signal of the photo-detector is reduced when raindrops accumulate on the outer surface of the windshield because they refract the light beam away from such photo-detector. The present invention differs by providing a mask in front of the photo-detector to prevent light from being transmitted directly from the light source to the photo-detector and refracting a portion of the light beam with the detected raindrops to the photo-detector. As a result the output signal of the photo-detector increases with increasing amounts of raindrops on the windshield. The light detector of Fukatsu et al. consists of a plurality of pairs of photo-detectors, each photo-detector of a pair being positioned behind either an infrared transparent strip or an infrared opaque strip with the outputs of said pair of photo-detectors being connected to a differential amplifier to measure the amount of rain accumulating on the windshield. This optical detector is more complicated, expensive and bulky. Also, it suffers from the problem of ambient light because changes in ambient light would effect the output signals of both photo-detectors of each pair. Finally, there is no way of differentiating from the light detection of raindrops on the outside surface of the windshield and the detection of fog on the inner surface of the windshield.
The optical detection method and apparatus of the present invention has several advantages over the above-discussed prior art, including the ability to monitor a much larger area of rainfall on the windshield so that the output signal of the photo-detector is more accurate in measuring small accumulations of randomly located droplets. In addition, by employing a mask to block light from being directly transmitted from the light source to the photo-detector and by employing light refraction from the raindrops to redirect the light to the photo-detector, the output signal of the photo-detector increases with increasing amounts of rain to provide more sensitive detection at the onset of rain. Also, the photo detection method and apparatus of the present invention is capable of detecting small amounts of rain in the presence of high ambient light and is not effected by changes in ambient light. The optical detection method and apparatus of the present invention also eliminates errors in the photo-detector output signal due to external factors unrelated to moisture, such as changes in temperature and aging of the LED light sources and photo-detector, power supply voltage variations or changes in ambient light by employing negative feedback through a reference light source. This reference light source sets the bias of the photo-detector to an operating point of high sensitivity to infrared light, and cancels any changes in the photo-detector output signal due to these external factors by negative feedback from the output of a tuned amplifier connected to the photo-detector transistor through a feedback circuit to the reference light source.